Interpretive Summary: The apple maggot fly is a major quarantine pest of apples in the western U.S. Efficient detection methods for adult flies are needed to tell if an area is free of or has a low prevalence of the fly. Personnel at the USDA-ARS Yakima Agricultural Research Laboratory in Wapato, WA and Washington State University, Cornell University, and the University of Notre Dame tested host fruit odor discrimination by apple maggot flies associated with black and ornamental hawthorn trees. It was found that apple maggot flies in black hawthorn tress oriented positively to black hawthorn odor lures and negatively to ornamental hawthorn odor lures, whereas the reverse was true of flies in ornamental hawthorn trees. Results suggest that fruit odor lures used for detecting apple maggot flies need to take into account the host tree species in order to be effective.

Technical Abstract:
Prezygotic isolation due to habitat choice is important to many models of speciation-with-gene-flow. Habitat choice is usually thought to occur through positive preferences of organisms for particular environments. However, avoidance of non-natal environments may also play a role in choice and have repercussions for postzygotic isolation that preference does not. The recent host shift of Rhagoletis pomonella (Diptera: Tephritidae) from downy hawthorn, Crataegus mollis, to introduced apple, Malus domestica, in the eastern U.S. is a model for speciation-with-gene flow. However, the fly is also present in the western U.S. where it was likely introduced via infested apples or hawthorns < 60 years ago or, less likely, was native. R. pomonella now attacks two additional hawthorns in the West, the native C. douglasii (black hawthorn) and the introduced C. monogyna (English ornamental hawthorn). Flight tunnel tests have shown that western apple-, C. douglasii-, and C. monogyna-origin flies all positively orient to fruit volatile blends of their respective natal hosts in flight tunnel assays. Here, we show that these laboratory differences translate to nature through field trapping studies of flies in the state of Washington. Moreover, western R. pomonella display both positive orientation to their respective natal fruit volatiles and avoidance behaviour (negative orientation) to non-natal volatiles. Our results are consistent with the existence of behaviourally differentiated host races of R. pomonella in the West. In addition, the rapid evolution of avoidance behaviour appears to be a general phenomenon for R. pomonella during host shifts, as the eastern apple and downy hawthorn host races also are antagonized by non-natal fruit volatiles.